Pierce and clinch nuts were first invented by the predecessor in interest of the assignee of this application over 50 years ago as shown, for example, by U.S. Pat. Nos. 3,089,360, 3,091,843 and 3,152,628. As shown in such patents, the original pierce and clinch nuts were T-shaped in cross-section including a projecting pilot portion having a threaded bore and integral flange portions on opposed sides of the pilot portion. Where the fastener is utilized as a pierce nut, the top face of the pilot portion is driven by a plunger against a panel supported on a die member or die button and the pilot portion pierces an opening in the panel and the panel is secured to the nut by the die member as disclosed, for example, in the above-referenced U.S. Pat. No. 3,089,360. Alternatively, the fastener may be utilized as a clinch nut, wherein an opening is preformed through the panel configured to receive the pilot portion and the panel is then secured to the fastener by the die button either by deforming the projecting end of the pilot portion or by deforming the panel into grooves in the pilot portion. As used herein, the term “self-attaching nut,” includes both clinch and pierce nuts.
Various improvements to self-attaching nuts or female fasteners have been made by the predecessor in interest of the assignee of this application, primarily to improve the push-off strength or retention of the fastener to the panel as disclosed, for example, in U.S. Pat. Nos. 3,648,747 and 4,971,499. The self-attaching female fasteners disclosed in these patents include a central pilot portion having a planar top face, a threaded bore through the top face, flange portions on opposed sides of the pilot portion each having a planar bearing face as described above; however, the panel bearing faces each include a “re-entrant groove” which provides significantly improved push-off strength when installed in a metal panel. As used herein, the term “re-entrant groove” includes opposed inclined side walls and a bottom wall, wherein the inner side wall is inclined outwardly from the bottom wall toward the outer side wall and the outer side wall is inclined inwardly toward the inner side wall forming a restricted opening to the grooves, wherein the width of the grooves adjacent the panel bearing or support faces of the flange portions are less than the width of the grooves adjacent the bottom wall. As disclosed in these patents, the bottom wall is flat and parallel to the panel support faces of the flange portions.
The method of installing the self-attaching nuts disclosed in these patents includes supporting a panel on a die button having projecting clinching lips and the pilot portion is driven by a plunger through an opening in the panel. The projecting lips or “clinching lips” of the die button are driven against the panel, wherein the clinching lips deform panel portions adjacent the panel opening into the grooves and against the bottom wall. Where the self-attaching nut is used as a pierce nut, the pilot portion pierces a slug from the panel, forming the opening through the panel as described above. However, in this embodiment, the end faces of the clinching lips deform panel portions adjacent the panel opening against the bottom wall of the groove deforming the panel portions inwardly against and beneath the inclined inner side wall of the grooves and outwardly against and beneath the inclined outer side wall of the grooves forming a more secure installation having greater push-off strength.
In a typical application of a self-attaching nuts of the types described above, the self-attaching nuts are fed to an installation head having a reciprocal plunger generally attached to the upper die shoe of a die press and the die button is installed in the lower die shoe of the die press. A panel is received on the die button and, upon closing of the die press, the plunger is driven against a back or bottom face of a nut, which drives the nut through a plunger passage into the panel to install the self-attaching nut in the panel as described above. The panel may be simultaneously formed in the die press and the die press may include multiple installation heads and die buttons to simultaneously install a plurality of self-attaching nuts in the panel. Various means are utilized to feed the self-attaching nuts female fasteners to the installation head. For example, the self-attaching nuts may be fed to the installation head in bulk or the fasteners may be interconnected by frangible connector elements in a continuous strip of fasteners, wherein the plunger simultaneously shears a fastener located in the plunger passage from the fastener strip and installs the fastener in a panel as described in U.S. Pat. No. 3,845,860, also assigned to the predecessor in interest of the assignee of this application. In a preferred embodiment, the frangible connector elements are frangible wires, which may be formed of any suitable frangible material, which are installed in linear grooves in the bottom face of the fasteners on opposed sides of the fastener bore, preferably aligned with the fastener grooves as also disclosed in the above-referenced U.S. Pat. No. 4,971,499.
As will be understood by those skilled in this art, the self-attaching female fasteners described above may be formed in a rolling mill, wherein the desired cross-sectional shape of the self-attaching female fastener is rolled from a steel wire, the bores are punched or pierced through the pilot portion, the fasteners are cut to length and the bore is typically tapped or internally threaded. However, it is difficult to accurately roll the desired shape of the re-entrant groove because the fastener strip must be very accurately located relative to the forming rolls and it is presently not possible to balance the rolling forces in a conventional self-attaching nut configuration. Where the self-attaching fasteners are to be interconnected by frangible connector elements in a strip form, as described above, the wire grooves are rolled into the bottom face of the strip during roll forming of the fasteners and wires are then inserted into the grooves and secured.
As will also be understood by those skilled in this art, one limitation upon applications for self-attaching female fasteners of the type described is panel thickness. That is, the self-attaching female fasteners described above have a limited range of panel thicknesses which will provide a good installation. Further, although the self-attaching female fasteners having a re-entrant groove described above provide excellent push-off strength, further improvements can be made, particularly for heavy metal applications. It is also desirable to substantially completely fill the re-entrant grooves for improved retention.
Another problem associated with self-attaching nuts of the types described above results from the tapping of the nut bore. A female threaded bore is formed by piercing or punching an opening through the central pilot portion and then the bore is internally threaded with a conventional tapper, typically received through the top face of the pilot portion. As will be understood by those skilled in this art, the tapping operation results in burrs which extend from the threaded bore above the back or bottom face of the nut. As stated above, the self-attaching nut is then installed in a panel by driving a plunger against the bottom face of the nut, which drives the burrs back into the threaded bore of the nut sometimes interfering with the receipt of a male threaded fastener, such as a bolt, following installation of the self-attaching nut in a panel. This problem is presently solved by forming a countersink in the bore at the bottom face of the nut which removes the tapping burrs from the bottom face. The countersink is typically a frustoconical opening in the bore at the back face of the nut. As will be understood, however, this requires a separate operation which may slow the manufacturing line and thus adds additional cost to the self-attaching fastener.
There has thus been a long-felt need for a solution to the problem of tapping burrs being driven back into the threaded nut bore during installation by a plunger without adding cost to the self-attaching nut, such as by forming a countersink or counterbore in the threaded nut bore in the bottom face of the nut. The self-attaching nut of this invention eliminates this problem without substantial additional cost as now described.